The problems solved by the present invention are best explained by reference to a conventional electronic driver circuit 10 as shown in FIG. 1. The illustrated driver has an output transistor 12 driving a load which, in this case, is a solenoid coil 14. Another transistor 16 is coupled between ground and the other end of the coil 14, with a zener diode 17 coupled between the collector of transistor 16 and ground. An input transistor 18 receives a positive going input pulse via a divider consisting of resistors 19 and 21. The input pulse turns the transistor 18 on, as well as turning on the transistors 12 and 16 (via another resistor 23), thereby permitting current to flow in the coil 14.
A protective transistor 20 is coupled to the transistor 12 to limit the maximum level of current that the transistor 12 may conduct. Also, the base of the transistor 20 is coupled via resistors 22 and 24 to the load 14. With this arrangement, a defect in the load, such as could result in a low potential at node 26, would allow increased base drive to the transistor 20 to turn it more fully on, resulting in the transistor 12 being turned fully off to protect it from damage due to excessively high currents.
A capacitor 28 is coupled to the junction between resistors 22 and 24 to prevent transient, low level signals (not true faults) from turning on the transistor 20, and also to prevent transistor 20 from turning on before transistor 12 turns on when transistor 18 receives an input pulse.
A problem with the illustrated driver can occur when the input transistor 18 needs to be turned on and off rapidly, Assume, for example, that the output transistor 12 has just been turned off, and that the voltage at node 26 is driven low by the normal flyback action of the coil 14. If the input transistor 18 is turned on before the low potential at node 26 has dissipated, the transistor 20 turns on hard because the low potential at node 26 provides base drive via the resistors 22 and 24. Consequently, the transistor 20 latches the output transistor 12 off when no real fault condition exists. This type of operation is clearly undesirable.